1. Field of the Invention
The present invention relates generally to horizontal bandsaw machines in which a saw head assembly carrying a bandsaw blade is lowered during a cutting operation in order to feed the bandsaw blade into an underlying material to be cut, and more particularly pertains to methods and apparatus for controlling the feeding of the bandsaw blade in horizontal bandsaw machines.
2. Description of the Prior Art
Horizontal bandsaw machines conventionally comprise a saw head assembly in which a flexible endless bandsaw blade is trained around a pair of wheels so that it may perform cutting operations when the wheels are driven. The saw head assembly is so arranged as to be raised and lowered with respect to an underlying base supporting the material to be cut. This movement can occur around a hinge pin or along a single or plural guide posts in response to operation of a driving means such as a hydraulic motor. Thus, in cutting operations, the saw head assembly is lowered from the raised position toward the base so as to feed the driven bandsaw blade into the material to be cut which has been placed on the base. Also, in most prevailing horizontal bandsaw machines, the saw head assembly is so arranged as to be raised by a hydraulic motor of a cylinder type when the hydraulic fluid is supplied thereinto and to be lowered by its own gravity when the hydraulic fluid is drained from the hydraulic motor. In this connection, it is desired that the upper limit of the up-and-down movement of the bandsaw blade is made a certain distance above the top of the material to be cut so as to prevent the bandsaw blade from being damaged by the material being fed, since most of materials are not exactly straight.
In such horizontal bandsaw machines, it is desired to feed the bandsaw blade into the material to be cut so that the cutting action is performed always at a certain stated or constant optimum cutting rate which is defined as the area (square millimeter) of cut per unit of time. Since many materials such as round bars and shape steels vary in cross-sectional length as cutting progresses therethrough, it is necessary to feed the bandsaw blade in correlation with this varying cross-sectional length of the material being cut so as to enable the bandsaw blade to operate continuously at an optimum cutting rate. Of course, if the bandsaw blade is not fed at an optimum cutting rate, it will work too hard and will become worn prematurely or otherwise it will work inefficiently.
For the purpose of performing cutting operations at a stated cutting rate according to the cross-sectional length of the material being cut, the bandsaw blade is conventionally arranged to be fed with a constant load or feeding force under the so-called load control or pressure control. In order to maintain the load or feeding force constant under the load control, the pressure of the hydraulic motor for the saw head assembly is controlled while the saw head assembly is lowered to feed the bandsaw blade into the material to be cut. The load control is intended to maintain the cutting resistance constant so as to feed the bandsaw blade at a constant cutting rate, since the cutting rate is generally proportional to the cutting resistance. Thus, under the conventional load control, the feeding speed of the bandsaw blade is controlled as a function of the cross-sectional cut length of the material in order to maintain a constant cutting resistance, while the driven speed of the bandsaw blade is kept unchanged.
The conventional load control, however, is defective especially in that it is based on the premise that the cutting resistance per unit of feeding amount or feeding speed of the bandsaw blade is always stated or constant. In fact, the cutting resistance per unit of feeding amount of the bandsaw blade has a tendency to increase as the feeding amount decreases. Accordingly, when the feeding amount is decreased in order to maintain the cutting resistance constant to respond to an increase of the cross-sectional length of the material being cut for instance, the cutting resistance per unit of feeding amount will have a tendency to further increase. The cutting resistance per unit of feeding amount will markedly increase, especially when cutting difficult-to-cut materials such as stainless steels which have to be cut at a low feeding speed or by a small feeding amount. Thus, the difficult-to-cut materials can be not cut at a uniform optimum cutting rate by the conventional load control in the horizontal machine, although the easy-to-cut materials which can be cut at a higher feeding speed or by a larger feeding amount can be cut at a substantially uniform cutting rate under the conventional load control.
Furthermore, the conventional load control has been also defective in that it is based on the premise that the cutting resistance will increase or decrease only in response to changes in the cross-sectional length of the material being cut. However, the fact is that the cutting resistance will increase as the bandsaw blade becomes worn regardless of the cross-sectional length of the material being cut. Thus, as the wear of the bandsaw blade increases during cutting operations under the conventional load control, the feeding speed of the bandsaw blade will decrease without any increase of the cross-sectional length of the material being cut with the result that the cutting rate will decrease.
In addition to the above, it is desired in horizontal bandsaw machines that the bandsaw blade be brought into contact with the material to be cut in the shortest idle time possible and then fed into the material at a constant optimum cutting rate, since the bandsaw blade is raised a certain distance above the top of the material as has been described hereinbefore.